Like with any building project, building a car requires creating a plan, sourcing the right parts and materials, and knowing how to put those pieces together. Individuals can purchase kits that include most, if not all, of the necessary parts along with detailed instructions.
Building a car from scratch requires a significant investment of both time and money. Building a new car from scratch requires in-depth design/engineering work and sourcing fabricators for various custom parts that are needed. Fabricating sheet metal, fiberglass and other materials into body parts can be very costly. If custom mechanical pieces are needed, expect the costs to rise even more drastically.
Due to the high volume of after-market parts manufactures, oftentimes vehicles can be built from sourcing various parts that come from different makers but fit and work together. These vehicles often are referred to as "Franken-cars."
Several manufacturers sell "kit cars," which are vehicles that require assembly by the purchaser. It is not uncommon for purchasers of kit cars to just bring the whole "kit" to a reputable mechanic to have the mechanic assemble the kit for them. Buyers of these kits should read the descriptions carefully, as sometimes the kit only includes cosmetic changes, and buyers are expected to purchase a "donor" car of a particular make or model to use as a base for the kit.